The major aims of the research are to trace the development of the prosocial behaviors of children under 3 and to inquire into their acquisition. Of these behaviors, caregiving and helping first command attention; these build on our previous studies of sharing and friendliness. Their ontogeny is charted in cross-sectional and longitudinal studies, in the field and the laboratory. The effects of such subject variables as sex, birth order, and peer group experiences are examined. The processes by which the behaviors are acquired are studied by experimental manipulations of an adult's (primarily the parent's) behavior; the manipulations include the effects of modeling, verbal prompting and instructing, and social reinforcement. As a by-product, the research should yield the beginnings of a much-needed scale of social development, emphasizing the positive, rather than the "negative" achievements of the early years. Furthermore, the theoretical implications of the findings are considerable. Inasmuch as no current theory predicts a prosocial infant, the formulation of a theory that does account for the development of prosocial behavior in children so young is anticipated as an outcome of these investigations.